Variable sensitivity speed circuit for variable pitch propellers



Aug. jz6, 195s l c. B. BRAHM 2,849,072 VARIABLE SENSITIVITY SPEEDCIRCUIT FUR VARIABLE PITCH PROPELLERS Filed June 23, 1955 ,een #masks/50/N VE N T 0R CHARLES B. BRA/-IM amos afm-E wm/65 By 7'/ ,sans a@DGEEE/J-c. ATTORNEY nited States Patent VARIABLE SENSiTI'i/ITY SPEEDClRtCUllT FOR VARIABILE-PITCH PROPELLERS Charles B. Brahm, WindsorLocks, Conn., assigner to United Aircraft Corporation, East Hartford,Conn., a corporation of Delaware Application June 23, 1955, Serial No.517,544 11 Claims. (Cl. 170--16.2)

This invention relates to a propeller control mechanism and particularlyto a speed control having variable sensitivity to minimize hunting.

An object of this invention is a device which will reduce thesensitivity of propeller control mechanism for relatively small speederrors.

A further object is mechanism limiting the value of arate-of-change-responsive signal opposing the signal producing thechange.

A further object is an electric speed responsive propeller pitch controlhaving a rate-of-pitch-change responsive mechanism opposing said speedresponsive control and having means limiting the effect of saidmechanism.

A still further object is a control mechanism having a low sensitivityaround the control point and a higher sensitivity when approaching thecontrol point.

Fig. l is a schematic drawing showing a controllable pitch propellerwith speed responsive control mechanism incorporating a rate responsivemodifying circuit.

Fig. 2 is a chart showing the effect of the sensitivity varyingmechanism.

In the speed control of propellers it has been found that reducing thesensitivity of the speed error signal producing mechanism as thepropeller approaches the on-speed position tends to reduce hunting andstabilize the system. However, in order to maintain proper control forlarge speed errors, it is necessary to utilize the full sensitivity ofthe control, so as to provide a large correcting force for a large speederror.

In the embodiment chosen to illustrate this invention a speed errorsignal indicating the extent of variation from a non-speed condition isproduced by electrical means, and after being amplified is utilized toproportionally actuate a hydraulic pitch changing mechanism to regulatethe propeller pitch and thus control the speed of the propeller-drivingengine by controlling the load on the engine. The rate of pitch changefor each unit of speed error is made great enough to prevent materialspeed variations from the selected or datum speed, but such a raterequires a control so sensitive that the mechanism will be subject tohunting or overshooting. In order to reduce or prevent such hunting, thesensitivity of the control, for small speed changes, is reduced bysupplying a signal opposing the speed error signal. The opposing signalis a signal responsive to the rate of propeller pitch change, which willreduce the sensitivity of the control for small speed changes.

In order to provide suilicient sensitivity for large speed errors andrapid pitch changes a limiting mechanism is placed in the rateresponsive signal circuit which will limit the amount of the opposingrate signal to a predetermined amount. Hence, for small speed errorswhich will give a comparatively slow rate of pitch change the opposingrate signal may be a relatively large proportion of the error signal tolthus reduce the effective pitch changing signal, or in other words,reduce the sensitivity of the circuit. -For large error signals `whichwill produce a large rate of pitch change the opposing signal is limitedin amount so that the opposing effective rate signal will be a smallproportion of the error signal which will give a circuit or increasedsensivity.

In the embodiment illustrated an engine l drives a well known type ofpropelle;y such as a controllable pitch propeller l2 having ahydraulically actuated piston 14 actuating cam mechanism 16 geared tothe several propeller blades 18, only one of which is shown, to changethe blade pitch. A shaft 20 geared to the blades 18 transmits the blademotion and position through gearing, including a differential gearing 22to a gear 24 located outside of the propeller. The `differential gearing22 may be of any desired type suitable for transmitting motion from arotating part to a non-rotating part so that the nal gear of the train`will move in accordance with the movement of the initial gear of thetrain with respect to its rotating support. Such a gear mechanism isshown and described in more detail in the patent to Longfellow et al.2,664,960 issued January 5, 1954, and patent to Richmond 2,666,960issued January 19, 1954, to which reference may be made for furtherdetails of the gear train. Gear 24 which is the last gear of the trainis mounted to rotate around a stationary shaft 26 and is actuated bymovement of the planet gear assembly 28. Gear 24 meshes with a gear 36connected with an internally threaded sleeve 32 held against axial orlongitudinal movement by shoulders 34 and stationary ring 36. Anexternally threaded member 38 meshes with the internal threads of member32 and is connected with a movable portion 4@ of a control valve 42.Valve 42 receives uid under pressure from a pump 44 taking fluid fromreservoir 46. Valve 42 selectively directs the fluid under pressure toone side or the other of piston 14 to control the propeller pitch.Threaded member 3S is internally splined to' receive a splined shaft 43held against longitudinal movement by collars 50 and stationary post 52.Shaft 48 is connected to gear S4 which is actuated by rack 56 connectedto a piston 5S which is hydraulically controlled by a speed responsivemechanism.

The speed responsive mechanism includes a propor-` tional solenoid 60actuated by speed responsive mechanism, presently to be described,controlling a valve 62 receiving pressure fluid from a pump 64 and areservoir 66 and selectively directing the pressure fluid to one side orthe other of the piston 58.

The mechanism thus far described operates in a well known manner tocontrol the propeller pitch and thus Control the engine speed. A speederror signal transmitted to the proportional solenoid titl will actuatevalve 62, piston 58, gear 54 and shaft 43 to turn the screw threadedmember and move valve member 4d longi- 1 tudinally to actuate the piston14 and change the propeller pitch. Changing of the propeller pitch willactuate shaft 20 and through the diferential gearing 22 actuate gear 24and gear 30 to turn the internally threaded sleeve 32 which being heldagainst longitudinal movement will return the valve member 40 to theshutoft' position shown in the drawings. Screw member 38 is held againstturning with the internally threaded member 32 by the splined shaft 48which is held against rotation except when motion is called for by theproportional solenoid 60.

A generator 68 driven by the engine l0 supplies alternating current to abridge 70 including a condensor 72 to provide an indication of speederror, three legs of the bridge being resistors and the fourth being acondensor. A condenser having a different reactance with differentfrequencies will unbalance the bridge upon changes in speed and providethe speed. error indication. A rectifier C) 74 along with filteringmechanism connected across one side of the bridge will give a negativesignal and a rectifier 76 connected across Vthe other side of the bridgeincluding the condenser 72 will give a positive signal. The two signalsare compared through resistors 78 and B to give a final error at 82which will be negative for overspeeds and positive for underspeeds.

The convention used with respect to the rectifier showing is that thearrow indicates current flow from plus to minus as opposed to electronfiow.

The speed error signal is fed between resistors 83 and 84 where it istransformed to a correction signal and fed to the well known chopperamplifier 86 where the correction signal is amplified. The amplifiedcorrection signal is then fed to the proportional solenoid 60 to startthe mechanism in motion to change the propeller pitch.

In the mechanism thus far described, a signal and propeller pitch changemotion is produced which is proportional to the speed error. It has beenfound that, in a system sufiiciently sensitive to give a rate of pitchchange sufficient to keep the speed control within the desired limits ofspeed variation, the system is so sensitive when approaching theon-speed position that it will overshoot and hunt to an undesireddegree. In order to prevent this unstable and hunt condition whileretaining the sensitivity for large speed errors, mechanism has beenprovided to induce an opposing signal to the i amplifier 85 to thusreduce the sensitivity of the system at the control point.

This opposing or negative feedback signal is provided by arate-of-pitch-change responsive mechanism including a gear S8 driving ascrew threaded member 90 having a nut 92 threaded thereon and movabletherealong by rotation of the screw 90. Nut 92 carries a potentiometerarm 94 moving along a potentiometer 96 connecting a fixed voltage source98 with ground. Arm 94 is connected to a resistor 100 and a condenser102 to a point between resistors 104 and 106 connected with the chopperamplifier 86. A pair of opposed rectifiers 103 and 110 are connected inparallel with one side connected to ground and the other side connectedbetween condenser 102 and resistors 104 and 106. Movement of arm 94being geared directly to the blades 18 will move at a rate proportionalto the rate of change `of the propeller pitch and will therefore changethe potential on -condensor 102 at a rate proportional to the rate ofchange of the propeller pitch. The signal sent through condensor 102 toresistors 104 and 106 opposing the speed error signal will, therefore,be proportional to the rate of change of the propeller pitch. This isdesirable for small pitch changes near a control point but it wouldreduce the sensitivity of the system for large pitch changes. In orderto prevent such a sensitivity reduction for large pitch changes,rectifiers S and 110 will conduct ofi any voltages generated above apredetermined voltage. The voltage limit is determined by the contactpotential of the rectifiers 108 and 110 and may be increased by puttingseveral rectifiers in series so that any desired voltage limit may beobtained. With the rectifiers I0?, and 110 in the circuit any voltagewhich may be created by rapid pitch changes will be limited to thepredetermined selected voltage of the rectifier deadband. Thus limitingthe opposing voltage obtainable for rapid pitch changes will retain thesensitivity of the circuit for rapid pitch changes since the limitingopposing voltage will be a very small fixed portion of the speed errorvoltage for large errors.

On the other hand, the opposing or feedback voltage for small speederrors will be a variable voltage and will be a much larger proportionof the speed error v-oltage and will therefore reduce the sensitivity ofthe system for small errors or, in other words, for small errors thespeed error signal opposed by the rate of change signal will result inan output signal to the proportional solenoid 60 which is a much smallerproportion of the speed error signal than would be the case for largespeed errors.

The effect of this signal limit mechanism can be shown graphically. Fig.2 is such a graph. It will be understood that the blade-angle changerate, or the number of degrees per second of blade angle pitch change,bears a direct relation to the strength of the signal output of theamplifier 86. The graph shows R. P. M. error as the X axis and bladeangle change rate, or degrees per second as the Y axis. The dotted lineindicates the rate at which the blade angle would change for anyparticular speed error without any feedback and it will be noted thatthe ratio remains substantially constant throughout the entire range.

rIhe full lines indicate the rate with the limited feedback connected inthe circuit. From this graph it will be noted that for small speederrors, the rute of pitch change is small, or in other Words, thesensitivity of the system is low. However, after a selected speed erroris reached, at which point the feedback signal is limited, then the rateof pitch change will increase with an increase speed error in the samemanner that it increased before the feedback system was applied. lt istherefore apparent from this graph that the system will have a lowsensitivity for small speed errors around the control point but willhave much higher sensitivity when approaching the control point from amaterial speed error or variation.

Another feature of the mechanism shown is that for extremely smallvariations in speed, the speed error signal is first transmitted to theamplifier and then tov the pitch change mechanism and it is not until apitch change is effected that an opposing signal is produce and fed tothe amplifier to oppose the speed error signal so that for very smallspeed errors the full amplification or sensitivity is obtained. In otherwords, the back-lash or lost motion in the mechanism from the speederror detector to the blade actuated potentiometer will allow fullsensitivity until motion is imparted to the potentiometer arm. This willtend to quickly correct any speed errors and prevent any material speedvariation excursion.

It is to be understood that the invention is not limited to the specificembodiment herein illustrated and dcscribed, but may be used in otherways without departure from its spirit as defined by the followingclaims.

I claim:

l. In a control System for a controlled device receiving an error signalvarying in accordance with the extent of the error to be corrected,means transforming said error signal into a proportional correctionsignal said controlled device including mechanism making a correction ata rate proportional to the value of the correction signal, meansproducing an opposing signal proportional to the correction rate, meanscombining said opposing signal with said correction signal in oppositionthereto, and means limiting the value of the opposing signal to apreselected fixed value.

2. In a speed control system, means creating a speed error signalproportional to the deviation of the speed of the controlled device froma selected speed datum. means transforming the speed error signal into aspeed changing movement of the conrtollcd device for returning the speedof the device to the datum speed and eliminating the speed error, andmeans creating a feedback signal, opposing said error signal,proportional to the rate of changing movement of the controlled device,and means limiting the value of said feedback signal to a predeterminedamount when said rate exceeds a predetermined value.

3. In a enginefpropeller speed control system, means creating a speederror signal proportional to the deviation of the engine speed from aselected speed datum, means transforming said error signal into pitchchanging movement of said propeller to vary the load on the engine andreturn the engine speed' to the datum speed and eliminate said speederror, and means responsive to the rate of pitch change creating afeed-back signal, means connecting said feed-back signal in oppositionto said speed error signal to reduce the signal fed to said transformingmeans, and means preventing said opposing feed-back signal fromexceeding a preselected value when said created feedback signal exceedssaid value.

4. A control for a controllable pitch propeller cornprising speedresponsive means, pitch changing means and means connecting said speedresponsive means with said pitch changing means including means changingsaid pitch at a rate proportional to the speed error, and meansresponsive to the rate of pitch change reducing the proportion of thepitch change rate to the speed error and meansv effective above aselected rate of pitch change less than the maximum rate of pitch changelimiting the reducing effect of said pitch change responsive means to apreselected value.

5. Pitch control means for a controllable pitch propeller comprisingspeed responsive means including means creating a speed error signalhaving a value proportional to the speed error, hydraulically actuatedmeans responsive to said speed error signal for changing the propellerpitch to eliminate the speed error creating said error signal, meansresponsive to the rate of pitch changing movement creating a rate signalopposing said speed error signal to reduce the sensitivity of thecontrol means and means connected with said rate signal creating meanspreventing said opposing signal from exceeding a preselected value whensaid rate signal exceeds said value.

6. Propeller pitch control means comprising hydraulic pitch changingmeans, means creating an electrical speed error signal proportional tothe variation of said propeller from a datum speed, an amplifierproducing an amplified signal from said error signal, electro-magneticmeans actuating said hydraulic pitch changing means proportional to saidamplified signal, means creating an electrical rate signal proportionalto the rate of pitch change, means feeding said rate signal to saidamplifier in opposition to said error signal, and means limiting thevalue of said opposition signal when said rate signal exceeds apreselected Value.

7. A control as claimed in claim 6 in which the limiting means is a pairof opposed rectiers connected in parallel grounding said rate signal forall values above the contact potential of said rectiliers.

8. In combination with a propeller having pitch control mechanism, meanscreating a pitch changing signal, follow-up mechanism actuated by bladepitch changing movement eliminating said signal, electrical mechanismcreating a speed error electrical signal, electrical mechanism actuatedby said pitch changing mechanism creating a pitch change rate electricalsignal, rectiers limiting the value of said rate signal, means combiningsaid two electrical signals in opposition, V:deans amplifying saidcombined signals7 and means responsive to the polarity and value or'said amplified signals actuating said means creating said pitch changingsignal.

9. In a propeller control system means transforming an electrical errorsignal into a pitch changing signal said transforming means having aselected ratio of input to output, means reducing the effecting ratio ofsaid transforming means for relatively small error signals comprisingmeans creating an electrical signal responsive to the rate of pitchchange opposing said error signal, and means restoring said effectingratio for relatively large error signals comprising means limiting thevalue of said opposing electrical signal to a predetermined valuecorresponding to a selected rate of pitch change.

10. In a control system for a controlled device having a substantiallyfixed datum and subject to deviation from said datum, means creating anerror signal dependent on the extent of deviation of said controlleddevice from said datum, means transforming said error signal into acorrection signal, means responsive to said correction signal at a rateproportional to the value of said correction signal for eliminating thedeviation causing said error signal, said transforming means having aselected substantially constant ratio of correction signal to errorsignal and means reducing said correction signal by amounts proportionalto said rate when the rate is below a selected value, and meanspreventing the value of said reducing means from exceeding apredetermined amount at all rates above said selected value.

ll. A control for a controllable pitch propeller comprising speedresponsive means, pitch changing means and means connecting said speedresponsive means with said pitch changing means, including meanschanging said pitch at a rate proportional to the speed error, meansresponsive to the rate of the pitch change reducing the proportion ofthe pitch change rate to the speed error and means effective above aselected rate of pitch change between zero and the maximum ratepreventing the reducing elr'ect of said rate responsive means fromexceeding the value attained at said selected rate.

References Cited in the le of this patent UNITED STATES PATENTS2,382,847 Baumann Aug. 14, 1945 2,455,364 Hays Dec. 7, 1948 2,656,498Goodwin a Oct. 20, 1953 2,669,312 Dinsmore et al. Feb. 16, 1954

